where \(W\)  is the strain-energy function per unit volume. For the isotropic solids under consideration, \(W\)  can be assumed to be a function of the three principal stretches \(\lambda _1, \lambda _2, \lambda _3\) . Then the moduli can be computed according to the following formulae [1]}:
\({\cal A}_{iijj}&=&\lambda _i\lambda _jW_{ij}, \;\;\;\; \hbox{no summation on}\; i \; \hbox{or}\; j, \nonumber \\{\cal A}_{ijkl}&=&\frac{\lambda _i^2}{\lambda _i^2-\lambda _j^2}(\lambda _iW_i-\lambda _jW_j)\delta _{ik}\delta _{jl} \nonumber +\frac{\lambda _i\lambda _j}{\lambda _i^2-\lambda _j^2}(\lambda _jW_i-\lambda _iW_j)\delta _{il}\delta _{jk}, \;\;\;\;\;i\ne j,\nonumber \) 
